Fish Belt Transect (BLT) Surveys

Fish BLT Surveys

The fish belt transect (BLT) protocol is the primary method used to quantify reef fish assemblages at UVS sites. Divers swim standardized belt transects, visually identifying and sizing all fishes observed within a defined area. This method yields robust estimates of fish abundance, biomass, and community structure across depth strata and regions (see SOP for full methods).

Fish BLT data are stored into three tables:

  • uvs.blt_stations — Metadata for each station, including depth, diver, effort, and summary metrics
  • uvs.blt_observations — Raw fish counts by species, size class, and transect
  • uvs.blt_biomass_by_taxa — Derived biomass by station and taxon, using length-weight relationships

All records are linked via ps_station_id, and species names are standardized using taxon_id from taxonomy.fish.

uvs.blt_stations

This table contains one row per depth-stratified fish survey conducted at a UVS site. Each row includes dive metadata, survey effort (e.g., number of transects, area surveyed), and ecological summaries such as richness, total abundance, and biomass. Stations are linked to uvs.sites via ps_site_id. The ps_station_id is generated by appending a standardized depth bin label to the ps_site_id.

Standard depth strata for UVS methods

To ensure consistency across sites and protocols, station depth (depth_m) is binned into standard strata using fixed thresholds. The binned depth label is then appended to the ps_site_id to generate a unique ps_station_id.

Depth strata and labels:

  • Supershallow: <= 6 m05m
  • Shallow: 7–14 m10m
  • Deep: >= 15 m20m

Examples:

  • If a fish transect is conducted at 5.8 m → station ID = FJI_2025_uvs_001_05m
  • If the same site has another transect at 14.7 m → FJI_2025_uvs_001_10m

This depth binning scheme ensures consistent stratification across analyses, regardless of slight depth variation between dives.

Table 1: Fields in the uvs.blt_stations table
Field Type Required Description
ps_station_id STRING true Unique station ID (ps_site_id_depth), e.g., CHL_2024_uvs_001_20m
ps_site_id STRING true ◂ Foreign key to UVS site table
exp_id STRING true ◂ Expedition ID (ISO3_YEAR)
method STRING true ◂ Survey method (uvs)
protocol STRING true Survey protocol (fish_blt) used at the station
divers STRING true Names of divers who conducted the transects (comma-separated)
date DATE true ◂ Survey date (YYYY-MM-DD)
time TIME true ◂ Survey start time (24-hour format)
depth_m FLOAT true Average depth of the station (m)
depth_strata STRING true Binned depth category: supershallow, shallow, or deep
region STRING true ◂ Region name
subregion STRING true ◂ Subregion name
locality STRING false ◂ Optional local feature (e.g., reef, bay, cove)
n_transects INTEGER true Number of fish transects completed at the station
transect_length FLOAT true Length of each transect (m)
transect_width FLOAT true Width of each transect (m)
survey_area_m2 FLOAT true Total area surveyed at the station (m²)
habitat STRING true ◂ Habitat type (see controlled vocabulary)
exposure STRING true ◂ Exposure type (see controlled vocabulary)
n_species INTEGER false Species richness (optional summary)
abundance_m2 FLOAT false Fish abundance (individuals/m², optional summary)
biomass_gm2 FLOAT false Fish biomass (grams/m², optional summary)
notes STRING false Optional comments or QA notes
uvs.blt_observations

This table contains one row per fish observation recorded on an individual transect during a belt transect survey. Observations represent raw field data collected by divers and include species identity, estimated size, and count. Each row corresponds to a unique combination of species, size class, and transect.

  • Linked to the uvs.blt_stations table via ps_station_id
  • Taxonomic identity is standardized using accepted_name and aphia_id, which join to taxonomy.fish
  • Fish size (length_cm) and abundance (count) are used to calculate derived metrics (e.g., biomass) in downstream summaries
Transect dimensions by fish size
  • Fish ≥ 20 cm are recorded along a 25 m × 4 m belt (100 m²)
  • Fish < 20 cmare recorded along a 25 m × 2 m belt (50 m²) Note: When transects deviate from standard dimensions, actual surveyed area should be recorded and used for calculations.
Table 2: fish.observations Table Schema
Field Type Required Description
obs_id STRING true Unique observation ID from diver datasheet (e.g., CHL_2024_blt_AMF_0001)
ps_station_id STRING true ◂ Foreign key linking to uvs.blt_stations
diver STRING true Name of the diver who recorded the observation
station_label STRING false Field-assigned depth stratum label (e.g., shallow, deep)
transect STRING true Transect label: A, B, or C; OFF for observations recorded outside standard transects
depth_m FLOAT true Depth at which the transect was conducted
accepted_name STRING true Scientific name (Genus species) of observed taxon
aphia_id STRING true Unique AphiaID from WoRMS — links to taxonomy.fish
length_cm FLOAT true Estimated total length (cm) of individual or size class
terminal_phase BOOLEAN false TRUE if individual was identified as terminal phase (e.g., parrotfishes)
count INTEGER true Number of individuals observed in this size class
notes STRING false Optional comments or QA notes
Off-transect observations

Fish observed outside of standard transects (e.g., large pelagics, megafauna) are recorded using transect = "OFF". These are included for completeness but should be treated separately in standardized analyses.

uvs.blt_biomass_by_taxa

This table contains station-level summaries of reef fish biomass and abundance by species, derived from raw BLT transect observations. It is the primary analysis-ready table for generating summaries, figures, and community structure plots.

Each row represents a unique combination of ps_station_id and species, with metrics standardized to g/m² (biomass) and ind./m² (abundance). All values are computed using species-specific length–weight parameters from the taxonomy.fish table.

Taxonomic and ecological fields — including family, trophic group, and functional group — are joined from the central species reference and enable flexible aggregation for trophic or functional analyses.

Table 3: uva.blt_biomass_by_taxa Table Schema
Field Type Required Description
ps_station_id STRING true ◂ Foreign key to uvs.blt_stations
exp_id STRING true ◂ Expedition ID (ISO3_YEAR)
ps_site_id STRING true ◂ Site ID for spatial grouping
region STRING true ◂ Region name
subregion STRING true ◂ Subregion name
depth_strata STRING true Binned depth category: supershallow, shallow, or deep
depth_m FLOAT true Average depth of the station (m)
aphia_id STRING true ◂ Species ID (linked to taxonomy.fish)
accepted_name STRING false Scientific name (Genus species)
family STRING false Taxonomic family (e.g., Acanthuridae)
trophic_group STRING false Ecological trophic group (e.g., piscivore, herbivore)
functional_group STRING false Functional role (e.g., browser, planktivore)
count INTEGER false Total number of individuals observed
abundance_m2 FLOAT false Average density (individuals per m²)
biomass_gm2 FLOAT false Average estimated biomass (g/m²)